1 Technical Field
The present invention relates generally to stators for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators. More particularly, the invention relates to a stator for an automotive alternator which includes a multi-phase stator coil.
2 Description of Related Art
Conventionally, various types of stators for rotating electric machines have been provided for practical use.
For example, there is disclosed a stator for an automotive alternator in Japanese Patent Application Publication No. JP2010110199A. The stator includes a hollow cylindrical stator core having a plurality of slots arranged in its circumferential direction and a three-phase stator coil that is mounted in the slots of the stator core so as to have, as a whole, a hollow cylindrical shape.
Moreover, the stator coil is comprised of a plurality of phase windings, each of which is formed of a continuous electric conductor and includes a plurality of in-slot portions and a plurality of coil end portions (to be also referred to as bridging portions or turn portions hereinafter). Each of the in-slot portions is received in a corresponding one of the slots of the stator core. Each of the coil end portions is located outside the slots of the stator core so as to connect one circumferentially-adjacent pair of the in-slot portions which are respectively received in two different slots of the stator core.
In recent years, rotating electric machines for installation in a limited space, such as automotive alternators, have been required to have a smaller size and higher performance than ever before. Moreover, to reduce the size and improve the performance of a rotating electric machine, it is necessary to improve the space factor of the stator coil in the slots of the stator core and arrange the coil end portions outside the slots in alignment, small size and high density.
To meet the above requirement, there have been developed various methods and apparatuses for manufacturing stators. Among them, in terms of prioritizing manufacturing efficiency, a method is mainly used according to which the stator coil is first formed to have a predetermined shape and then mounted to the stator core.
However, in the case of using the above method, there are the following two problems.
First, due to the dimensional tolerances of raw materials and the manufacturing tolerances of the stator coil and the stator core, in mounting the stator coil to the stator core, an undesirable load may be imposed on the stator coil, thereby lowering the insulating properties of the stator coil. For example, the coil end portions of the phase windings of the stator coil may interfere with one other, thereby damaging insulating coats formed on the outer surfaces of the phase windings.
Secondly, there occurs variation in the mounting strength (or winding strength) of the stator coil to the stator core. In other cases, it is desired to set the mounting strength as needed. For example, when an abutting load higher than a threshold value is applied between the side surfaces of the electric conductors forming the phase windings of the stator coil and the inner wall surfaces of the slots of the stator core, vibration of the stator core, which is caused by magnetic force generating magnetic noise, may cause the stator coil to resonate therewith, thereby damaging the stator coil. In contrast, with a suitable abutting load applied between the side surfaces of the electric conductors and the inner wall surfaces of the slots, the rigidity of the stator core can be improved.
To solve the above first problem, one may consider increasing the thickness of the insulating coats of the electric conductors and widening the gaps between the coil end portions of the phase windings of the stator coil. However, in this case, the space factor of the stator coil in the slots of the stator core would be lowered and the axial length (or height) of the coil end portions would be increased. Consequently, it would become difficult to increase the output and reduce the size of the rotating electric machine.
On the other hand, to solve the above second problem, one may consider employing different types of stator coils according to the need. However, in this case, preparing a number of different types of stator coils would result in an increase in the manufacturing cost.